The present invention relates to a technology for acquiring stable receiving sensitivity by controlling an error rate of a received signal, and is applied to, e.g., an optical receiving unit of a wavelength multiplexing device in an optical transmission system.
FIG. 22 is a diagram showing an outline of a general type of conventional wavelength multiplexing optical transmission. In this case, an optical signal transmitted from an optical transmitting unit 91 is transmitted across on a transmission path via a wavelength multiplexing unit 92 and an optical amplifier 93, demultiplexed by a wavelength demultiplexing unit 94, then received by an optical receiving unit 95 and properly distributed to a device existing at a rear stage.
FIG. 23 shows a configuration of this optical receiving unit 95. The optical signal having an arbitrary wavelength, which is demultiplexed by the wavelength demultiplexing unit 94, is converted into an electric signal by a photoelectric converting unit 81, and is further converted by an electric signal processing unit 82 into a signal having a speed that facilitates signal processing. Then, this signal is, after an error has been corrected by an error correcting unit 83, converted into a signal (an optical signal in this example) for a subscriber line by a subscriber optical converting unit 84 and distributed to the subscriber side.
Further, FIG. 24 shows automatic identifying point control of keeping an optimum transmission quality by effecting feedback to an electric signal processing unit 82 from an error correction processing unit 83 in the optical receiving unit 95.
As shown in FIG. 24, the error correction processing unit 83 includes an error detecting unit 88 and an error correcting unit 89, wherein an error detected by the error detecting unit 88 is corrected by the error correcting unit 89. Moreover, the error correction processing unit 83 obtains a rate, as an error rate, of the error detected by the error detecting unit 88 to the transmission signal, i.e., as a quantity of deterioration of the transmission quality that is caused due to the transmission path, and feeds this error rate back to the electric signal processing unit 82.
The electric signal processing unit 82 has an identifying unit 85, a serial-to-parallel converting unit 86 and an optimum point control unit 87. The electric signal processing unit 82 identifies the electric signal given from the photoelectric converting unit 81, outputs, after the serial-to-parallel converting unit 86 has converted the signal, this converted signal to the error detecting unit 88, and at the same time controls voltage- and phase-directional optimum identifying points of the signal of the identifying unit 85 so as to minimize the error rate (the deterioration quantity), thus controlling the feedback control so as to attain optimum receiving performance at all times.
FIG. 25 is an explanatory diagram when the identifying unit 85 identifies “1/0” of an input signal. FIG. 25 shows a waveform of the input signal, wherein the axis of ordinates represents time, and the axis of abscissa represents a voltage.
FIG. 25(a) shows a waveform when an SN ratio declines due to the transmission path, wherein the signal originally taking a waveform depicted by bold lines comes to take a waveform distorted due to the decline in a range indicated by narrow lines.
The identifying unit 85 has a function of identifying this waveform in the voltage-direction and a function of identifying the waveform in the phase-direction. The identifying unit 85, in the case of identifying the waveform in the voltage-direction, if a voltage of the input signal exceeds an identifying voltage Vopt indicated by a dashed double-dotted line, identifies the waveform as “1” and, if not, identifies it as “0”, whereby a specified voltage corresponding to “1” or “0” is outputted.
Accordingly, a waveform subsequent to the voltage-directional identification takes, as shown in FIG. 25(b), a specified value in the voltage-direction but has a distortion (scatter) in the phase-direction.
Then, the identifying unit 85, in the case of making the phase-directional identification, identifies “1” or “0” when the input signal is in an identifying phase Popt indicated by a dashed double-dotted line, and, if identified as “1”, outputs specified rising and falling signals on the basis of the identifying phase Popt.
Namely, the waveform after making the voltage-and phase-directional identification becomes as specified both in voltage and in phase as shown in FIG. 25(c).
At this time, a proper identifying voltage and a proper identifying phase differ depending on a degree of distortion, etc. of the waveform, and hence an error rate is obtained in a way that makes different each of the identifying voltage and the identifying phase. The control (optimum point control) is conducted to obtain such an identifying voltage and an identifying phase as to minimize this error rate.
FIG. 26 shows a change in the error rate (an error rate characteristic) in the case of making different the identifying voltage and the identifying phase.
As shown in FIG. 26, the error rate characteristic plots a curve in which the error rate is minimized at such points that the identifying voltage and the identifying phase are optimized, and becomes larger as deviated more greatly from these optimum points. Then, when the SN ratio declines, the error rate is shifted on a larger side than when the SN ratio is enhanced.
Accordingly, the error rate is controlled to ERR2 when the SN ratio of the input signal declines and controlled to ERR1 when the SN ratio is enhanced. Note that ERR3 is a parameter determined by a capacity of the error correcting unit, and the error rate equal to or smaller than this threshold value ERR3 is required.                Patent document 1        Japanese Patent Application Laid-Open Publication No. H03-70223        Patent document 2        Japanese Patent Application Laid-Open Publication No. S63-221733        Patent document 3        Japanese Patent Application Laid-Open Publication No. H04-54043        Patent document 4        Japanese Patent Application Laid-Open Publication No. H09-326755        